AP_HAL_SITL: add discard_input method on UARTDriver

libraries/AP_HAL_SITL/UARTDriver.cpp

@@ -174,6 +174,12 @@ int16_t UARTDriver::read(void)
     return c;
 }
 
+bool UARTDriver::discard_input(void)
+{
+    _readbuffer.empty();
+    return true;
+}
+
 void UARTDriver::flush(void)
 {
 }

libraries/AP_HAL_SITL/UARTDriver.h

@@ -49,6 +49,8 @@ public:
     uint32_t txspace() override;
     int16_t read() override;
 
+    bool discard_input() override;
+
     /* Implementations of Print virtual methods */
     size_t write(uint8_t c) override;
     size_t write(const uint8_t *buffer, size_t size) override;